Mun Ho Kim

Synthesis of silver nanoplates with controlled shapes by reducing silver nitrate with poly(vinyl pyrrolidone) in N-methylpyrrolidone

This article describes a simple approach to the synthesis of Ag nanoplates with controlled shapes by reducing silver nitrate (AgNO3) with poly(vinyl pyrrolidone) (PVP) in N-methylpyrrolidone (NMP). The hydroxyl end groups of PVP have been shown to serve as a mild reductant for the kinetically controlled synthesis of Ag nanoplates. Here, we demonstrated that Ag nanoplates with tunable size and aspect ratio could be formed with high yields in a kinetically controlled synthesis using NMP as the reaction medium. NMP plays a critical role in the formation and growth of the nanoplate morphology. The reaction time and weight ratio of PVP to AgNO3 could both be adjusted to permit the shape-controlled synthesis of Ag nanoplates, that is, with control over the nanoplate size and aspect ratio. Shape control permitted tuning of the localized surface plasmon resonance (LSPR) peaks of these Ag nanoplates over the visible and near-IR regions.

Fabrication of robust, high-quality two-dimensional colloidal crystals from aqueous suspensions containing water-soluble polymer

In this letter, we report that two-dimensional (2D) polystyrene colloidal crystals of much higher quality can be fabricated in a single-step confined convective assembly process by introducing water-soluble polymer into the colloidal suspension. The presence of the water-soluble polymer in the aqueous phase enables the colloidal particles to arrange into more hexagonally close-packed single crystalline domains, and thus reduces the defect density. In addition, the physical stability of the final 2D colloidal crystals is improved because each particle is bonded to neighboring particles and to the substrate by polymer bridges.

Maneuvering the growth of silver nanoplates: use of halide ions to promote vertical growth

The unique shape and crystalline structure of Ag nanoplates provide an interesting model system for investigating the roles of capping agents in controlling the evolution of the nanostructure shape during growth. This article describes a simple approach to the synthesis of Ag nanoplates with well-controlled shapes in which halide ions (including Cl−, Br−, and I−) guide the well-defined kinetically controlled synthetic route. The presence of iodide ions promoted vertical growth in the nanoplate structure, resulting in small thick nanoplates. The time, during the nanoplate growth process, at which iodide ions were added could be adjusted to control the shapes of the Ag nanoplates by controlling the lateral and vertical dimensions of the nanoplates. This shape control method permitted tuning of the localized surface plasmon resonance (LSPR) peaks of the Ag nanoplates over the visible and near-IR regions of the spectrum.

Growth pathways of silver nanoplates in kinetically controlled synthesis: bimodal versus unimodal growth

Silver nanoplates were synthesized by reducing silver nitrate (AgNO3) with poly(vinyl pyrrolidone) (PVP) in N,N-dimethylformamide (DMF). In the synthesis, the hydroxyl end groups of the PVP served as a mild reductant in the kinetically controlled synthesis, and DMF played a critical role as a reaction medium (solvent) for the formation and growth of the nanoplates. In the present study, the growth of the nanoplates proceeded along different pathways, as evidenced by variations in the shape evolution, depending on the PVP to AgNO3 weight ratio. When the concentration of PVP is below a certain value, a large number of small nanoplates with different sizes were initially formed and then fused together along their lateral planes, leading to the formation of a secondary large nanoplate (bimodal particle growth). When the PVP concentration became higher and the surface capping was enhanced, the small nanoplates continued to grow individually without fusing (unimodal particle growth). This study not only advanced our understanding of the role played by PVP in the kinetically controlled synthetic reaction but also allows us to produce Ag nanoplates with a high aspect ratio in a single step.

Controlling Gaussian and mean curvatures at microscale by sublimation and condensation of smectic liquid crystals

Soft materials with layered structure such as membranes, block copolymers and smectics exhibit intriguing morphologies with nontrivial curvatures. Here, we report restructuring the Gaussian and mean curvatures of smectic A films with free surface in the process of sintering, that is, reshaping at elevated temperatures. The pattern of alternating patches of negative, zero and positive mean curvature of the air–smectic interface has a profound effect on the rate of sublimation. As a result of sublimation, condensation and restructuring, initially equilibrium smectic films with negative and zero Gaussian curvature are transformed into structures with pronounced positive Gaussian curvature of layers packing, which are rare in the samples obtained by cooling from the isotropic melt. The observed relationship between the curvatures, bulk elastic behaviour and interfacial geometries in sintering of smectic liquid crystals might pave the way for new approaches to control soft morphologies at micron and submicron scales.

Fabrication of periodic nanoparticle clusters using a soft lithographic template

A novel fabrication method has been developed for the preparation of a periodic array of nanoparticle clusters (NPCs) using a sublimable liquid crystal (LC) material. The defect structures of the LC film provide the specific topographical confinement to trap nanoparticles (NPs) and assemble the NPs to generate NPCs during thermal annealing. This system shows a simple regulation of the size of NPCs by varying the concentration of the NP-suspension. Additionally, an illumination system using quantum dots (QDs) is fabricated using the manipulation method reported here.

Creation of a superhydrophobic surface from a sublimed smectic liquid crystal

Dual-scale structures showing superhydrophobic characteristics have been fabricated using sublimable smectic liquid crystals (LCs). Here, toric focal conic domains (TFCDs) of smectic LCs were prepared on micron-sized square pillar patterns. And the layer by layer reconstruction of TFCDs under a thermal sublimation process was followed to form nano-scale hemi-cylinders. Based on this dual-scale roughness, the superhydrophobic surface was successfully created with a water contact angle (CA) of ∼150° and a low CA hysteresis of ∼9°. The resulting superhydrophobic surface is the first application using sublimable LCs, suggesting a new approach for potential applications in LC science and engineering.

Photoaging behavior of 3-D colloidal photonic crystals

Since most polymers are not inherently stable to light, their photochemical behavior remains a subject of constant interest. In this study, we investigated the photoaging behavior of polymeric colloidal photonic crystal films. As a consequence of photochemical reactions containing chain-scission, photoaging induced morphological changes in the film surface including changes in the size, surface roughness, shape, and packing structure of PS particles. These structural modifications came about the deterioration of optical properties (photonic bandgap) during photoaging. Based on our results and observations, a plausible correlation between the evolutions of the optical properties and the structural modifications during photoaging was established. Finally, the compositional modifications of the surface based on oxygen incorporation behavior caused by the photooxidation process are discussed based on the ATR-FTIR and XPS analyses.

Synthesis of hex nut shaped Au–Ag nanostructures via a galvanic replacement reaction and their optical properties

Galvanic replacement reactions have been successfully employed to produce hollow bimetallic nanostructures with a range of shapes. This paper presents a systematic study of the galvanic replacement reaction between 122 nm thick hexagonal Ag nanoplates and HAuCl4 in an aqueous medium. We monitored the morphological and spectral changes as a function of the HAuCl4-to-Ag molar ratio. The replacement reaction on the hexagonal Ag nanoplates resulted in the formation of a series of porous Au–Ag nanostructures with a hex nut shape. The size and density of the pores could be controlled by varying the molar ratio of HAuCl4-to-Ag molar ratio. The localized surface plasmon resonance (LSPR) peaks of these nanostructures were readily tuned from 494 nm to 1044 nm as the porosity of the product was increased.

One-Step Synthesis of Hollow Dimpled Polystyrene Microparticles by Dispersion Polymerization

The design and preparation of hollow nonspherical microparticles are of great significance for their potential applications, but the development of a facile synthetic method using only one production step remains a great challenge. In the current work, a new template-free method based on dispersion polymerization was successfully developed to produce anisotropic hollow polystyrene (PS) microparticles in a single step. In the synthesis, ammonium persulfate (APS) played a critical role in the formation and growth of highly uniform and stable hollow PS microparticles. By varying the concentration of APS and that of the stabilizer used, polyvinylpyrrolidone, we were able to control the average size of the PS particles and their degree of concavity. Based on our results and observations, a plausible mechanism for formation of these unusually shaped PS microparticles was proposed.